Reduction of the time for executing an externally commanded transfer of data in an integrated device

ABSTRACT

Cumulative delay contributions introduced by an input buffer and by the metal line that distributes the buffered external control signal to data transfer circuits for performing a transfer of data to and from an integrated device are reduced by having the external signal distributed unbuffered through a metal line of sufficiently large size. This introduces a negligible intrinsic propagation delay being within the specified maximum admitted input pad capacitance. The delay reduction is also based on locally dedicated input buffers for each data transfer circuit, and for applying thereto a buffered replica of the external signal present on the metal line.

FIELD OF THE INVENTION

The present invention relates in general to integrated circuits having input/output (I/O) buffers for communicating external the integrated circuits, and in particular, to a way of reducing the time needed for executing a transfer of data commanded from external the integrated circuits.

BACKGROUND OF THE INVENTION

If one considers the signal path that determines the transfer time of data from the instant the external command is generated to the valid transition on the output pad(s) of the data in a generic integrated circuit, as schematically depicted in FIG. 1, it is evident that the electrical characteristics of the signal path in the integrated circuit of certain externally applied commands influences the time taken to complete the operation (cycle). For example, the command may be an external clock signal (CLK) applied on a dedicated input pad of the integrated circuit.

In the sample diagram of FIG. 1, focusing attention on the production of data on an external data bus, it is evident that the output buffer enabling signal OEN and the timing signal CLK are sequential to the response time of the output buffer (Tkqv=time between CLK and DQ_value Valid). External signals AVD and CEN are sequential to enabling timing of the address buffers. Therefore, any propagation delay for all these external signals or commands contributes to determination of the random access time of the integrated device.

Typically, for a memory device, the access time can be divided in tree parts: INPUT time (delay between the instant at which a control/address pad assumes a valid value and the instant at which the corresponding internally propagated signal assumes a valid value on the relative internal circuit node), MAIN READ time (time taken to read the data from the array cells), and OUTPUT time (the time taken to transfer the read data to an external BUS).

FIGS. 2A and 2B illustrate certain features of typical internal propagation paths, and FIGS. 3A and 3B illustrate how the two parts, namely the OUTPUT time (or Tkqv) and the INPUT time of the access time are, in turn, sums of distinct delay contributions caused by internal circuit characteristics.

The following main contributions are graphically illustrated in the two FIGS. 3A and 3B:

-   -   T1=delay introduced by an input buffer;     -   T2=delay introduced by the connecting line; and     -   T3=delay introduced by an output buffer.

Analyzing the time Tkqv, the user generates a leading edge or front edge on the CLK_PAD when ready to accept new data from the device core that, by way of example, may be a nonvolatile flash memory.

Referring to the circuit diagram of FIG. 2B, the CLK signal needs to propagate through an input buffer. The properly dimensioned input buffer drives a metal line that distributes the buffered CLK signal to the flip-flops of all the output buffers. Finally, the data signals pass through the output buffers in order to be made available on the DQ_PADs.

In commercially available flash memory devices, the above-noted delay times are on the order of:

-   -   T1˜1.5 ns;     -   T2˜0.5 ns; and     -   T3˜5.0 ns.

Similar considerations may be made also for the internal propagation delays of external control signals CEN and AVD of the address data input buffers. Considering the circuitry to which the delay time T1 is attributed, even the minimal circuitry needed to implement an input buffer for a control signal issued by an external user device should at least include two cascaded inverters, one of which is either a NOR gate or a three-state inverter.

FIG. 4 respectively depicts a functional circuit diagram, a detailed electrical circuit diagram and the waveforms on the pad (A) on the intermediate node (B) between the two cascaded inverters and on the driven metal line (C).

The load driven by an input buffer is represented by the metal line that distributes the external control signal to a plurality of output data buffers or to a plurality of input address data buffers, and which has a non-negligible capacitance. This is because the cumulative gate load is driven as well as because the metal line is to be sufficiently large in order not to introduce an intolerable intrinsic delay (excessively resistive) in the propagation of the CLK signal.

Therefore, as a consequence, the two cascaded inverters that constitute an input buffer cannot be of minimal size but at least the second inverter needs to be dimensioned to be able to satisfactorily drive the relatively large load as noted above. However, increasing the size of the inverters could slow down the propagation of the signal onto the capacitive metal line (heavy load). This determines a waveform as indicated by the curve C at the bottom of FIG. 4.

It is evident that there is an opportunity of minimizing or reducing the above-noted delay contributions by achieving faster input buffering.

SUMMARY OF THE INVENTION

An object of the invention is to reduce the delay time from when a valid external control signal is produced on an input pad to when a corresponding valid internal signal is propagated to a plurality of synchronizing flip-flops or buffers to be enabled.

The cumulative delay contributions (T1+T2) introduced by an input buffer and by the metal line that distributes the buffered external control signal to a plurality of synchronizing and/or enabling circuits for performing a transfer of data can be significantly reduced by having the external signal applied on a pad distributed unbuffered through a metal line of sufficiently large size (conductivity) in order to introduce a negligible intrinsic propagation delay. This may be through fulfilling the specified maximum admitted input pad capacitance, and by realizing locally dedicated input buffers to each of a plurality of synchronizing and/or enabling circuits of data transfer of the integrated device for applying thereto a buffered replica of the external signal present on the distributing metal line.

It has been verified that the delay contribution (T2) due to the signal distribution metal line can be significantly reduced, in many cases practically halved, by forming the distributing metal line with a significantly increased size. Also, the delay contribution (T1) imputed to an input buffer for producing a buffered replica of the external signal applied on the pad may be even more sensibly reduced, often to about ⅓ or even ⅙, based on each locally formed small size input buffer driving a single flip-flop (FF) or a single enabling node of a controlled inverter. Also, there is no impact from a large parasitic capacitance because of the shortness of the metal connection to the controlled node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a generic integrated circuit with input and output data transfer structures according to the prior art.

FIGS. 2A and 2B show physical details of input and output decoupling and driving structures according to the prior art.

FIGS. 3A and 3B illustrate different cumulative delay contributions in inputting and outputting data according to the prior art.

FIG. 4 illustrates a typical ISD protection and decoupling of an externally applied signal and relative effects on the speed of transitions according to the prior art.

FIGS. 5A and 5B illustrate the I/O buffering architecture according of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The input/output architecture in accordance with the invention significantly reduces the delay contributions, thus improving both the data output speed as well as the input (access) times.

The architecture contemplates, as appreciated by those skilled in the art, the presence of ESD protection which, in case of parallel protection devices such as reverse biased junctions (diodes) as shown in FIGS. 5A and 5B, are commonly realized near the input pad. In case of serial type protection devices, they will be delocalized and replicated for the number of users of the input signal according to common practices, as also appreciated by those skilled in the art.

The size (width) of the low resistance distribution metal line, Fast Wire, needs to be compatible with the specifications of maximum tolerable input capacitance of the integrated device.

According to the buffering architecture, the power capabilities of the output buffers of the user device that issues the external control signal are exploited to charge the relatively heavy load constituted by the distributing metal line (Fast Wire) of the integrated circuit thus propagating the externally generated control signal very rapidly, without buffering it with an appropriately dimensioned input buffer. Differently from common practice, the necessary decoupling of the integrated circuit from the external world is implemented locally by a plurality of relatively small size (thus intrinsically fast) input buffers. Each buffer controls a respective flip-flop or enabling node of one of the two inverters constituting a local controlled buffer.

The output buffer of an external user already drives a relatively heavy load represented typically by a printed circuit board (PCB) metal line that could typically be on the order of 15-30 pF. This depends on the length of the connection line on the PCB. An additional capacitive load on the order of 0.5-1.0 pF, as constituted by an input signal distribution metal line of the integrated circuit, even if over dimensioned, hardly has any significance on the design of the output buffer of the external user. 

1-4. (canceled)
 5. A method of reducing time for executing a transfer of data in an integrated device controlled by an external device, the method comprising: distributing at least one external signal unbuffered through a distributing line of the integrated device; and locally realizing a decoupling input buffer for each respective data transfer circuit of the integrated circuit to locally produce a buffered replica of the at least one external signal.
 6. The method of claim 5, wherein the integrated device comprises at least one input pad connected to the distributing line and having an allowed input pad capacitance associated therewith, and wherein the distributing line has a width corresponding to the allowed input pad capacitance.
 7. The method of claim 5, wherein the at least one externally generated signal comprises at least one of a synchronizing signal and an enabling signal.
 8. The method of claim 5, wherein the integrated device comprises a nonvolatile flash memory device, wherein each data transfer circuit comprises a flip-flop and an output data buffer coupled thereto, and wherein the at least one externally generated signal comprises a clock signal and an enabling signal respectively applied to a clock input node of the flip-flop and to an enabling input node of the output data buffer.
 9. An input buffering and distributing structure for at least one externally generated signal applied to at least one input pad of an integrated device to be distributed to a plurality of data transfer circuits of the integrated device, the structure comprising: a distributing line connected to the at least one input pad for distributing the at least one external signal unbuffered; and a plurality of decoupling input buffers, each decoupling input buffer locally producing a buffered replica of the at least one externally generated signal for each respective data transfer circuit.
 10. The structure of claim 9, wherein the at least one externally generated signal comprises at least one of a synchronizing signal and an enabling signal for the plurality of data transfer circuits.
 11. The structure of claim 9, wherein the integrated device comprises a nonvolatile flash memory device, wherein each data transfer circuit comprises a flip-flop and an output data buffer coupled thereto, and wherein the at least one externally generated signal comprises a clock signal and an enabling signal respectively applied to a clock input node of the flip-flop and to an enabling input node of the output data buffer.
 12. The structure of claim 9, wherein the at least one input pad has an allowed input pad capacitance associated therewith, and wherein said distributing line has a width corresponding to the allowed input pad capacitance.
 13. An integrated device comprising: at least one input pad for receiving at least one externally generated signal; a distributing line connected to said at least one input pad for distributing the at least one externally generated signal unbuffered; a plurality of decoupling input buffers connected to said distributing line; a plurality of data transfer circuits connected to said plurality of decoupling input buffers; and each decoupling input buffer locally producing a buffered replica of the at least one externally generated signal for each respective data transfer circuit.
 14. The integrated device of claim 13, wherein the at least one externally generated signal comprises a synchronizing signal and an enabling signal for said plurality of data transfer circuits.
 15. The integrated device of claim 13, wherein the integrated device comprises a nonvolatile flash memory device, wherein each data transfer circuit comprises a flip-flop and an output data buffer coupled thereto, and wherein the at least one externally generated signal comprises a clock signal and an enabling signal respectively applied to a clock input node of said flip-flop and to an enabling input node of said output data buffer.
 16. The integrated device of claim 13, wherein said at least one input pad has an allowed input pad capacitance associated therewith, and wherein said distributing line has a width corresponding to the allowed input pad capacitance.
 17. An integrated device comprising: at least one input pad for receiving at least one externally generated signal; a distributing line connected to said at least one input pad for distributing the at least one externally generated signal unbuffered; a plurality of decoupling input buffers connected to said distributing line; a plurality of data transfer circuits connected to said plurality of decoupling input buffers, each data transfer circuit comprising an output data buffer coupled thereto; and each decoupling input buffer locally producing a buffered replica of the at least one externally generated signal for each respective output data buffer.
 18. The integrated device of claim 17, wherein the at least one externally generated signal comprises a cock signal.
 19. The integrated device of claim 17, wherein the at least one externally generated signal comprises an enabling signal.
 20. The integrated device of claim 17, wherein the integrated device comprises a memory device, wherein each data transfer circuit further comprises a flip-flop connected to said output data buffer, and wherein the at least one externally generated signal comprises a clock signal and an enabling signal respectively applied to a clock input node of said flip-flop and to an enabling input node of said output data buffer.
 21. The integrated device of claim 17, wherein said at least one input pad has an allowed input pad capacitance associated therewith, and wherein said distributing line has a width corresponding to the allowed input pad capacitance. 